Teardown and Repair: 20W Tubelight LED Driver

Tubelight Internals

Tubelight at home randomly stopped working this week for no apparent reason. Opened it up to see what went wrong. Started probing and saw that the input fuse (Supposedly a fuse but they put a 61Ohm resistor) was blown. Replaced that and the 41Ω, 1W resistor on the return path, and the tube light was back to normal. Not sure what caused the blow-up. Maybe the resistor degraded with time.

20W Tubelight Schematic and circuit
20W Tubelight Schematic and circuit

I went through and traced out the circuit for anyone wanting to know its schematic. The AC Input side is current limited with a “fuse” and fed into a classic full wave bridge rectifier(MB105) IC to convert to high voltage DC. There is space given in the input for filtration/safety capacitors but they have not populated it. Probably to save a few cents, might have put it there to pass the testing. The output of the rectifier contains a CY400 capacitor and Metal Oxide Varistors(MOV). This is fed to SIC9762 which is a high voltage LED driver with an integrated MOSFET. It’s a constant current LED driver with the drive current set at around 200mA with a sense resistor. On the output LED side, you have a large electrolytic capacitor of 100uF with a bleeding resistor to discharge the tube light immediately(else you will get a ghosting effect of the LED). There are 100 LEDs in total at the output.

All in all, it’s a penny-pinched circuit optimized to minimize the BOM to the lowest price possible. That’s exactly the reason you get these tube lights in the $3-$4 range per unit in the retail market.

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